If coins are tested in coin validators the inserted coins are tested for various properties to ensure a reliable discrimination of spurious coins. The properties determined for the coins also include their diameters. It is known to measure the diameter of coins by means of two photoelectric light barriers. The diameter thereof is calculated by means of the time the light barriers are covered and the time differential between the light barriers while the coins are passing. This way of size measurement has a measuring tolerance of about ±0.5 mm. However, sets or coins are known in which the individual coins do not differ by more than ±0.5 mm in diameter. There-fore, the known method to exactly identify counterfeit coins is inadequate.
It is further known to conclude on the diameter from the hiding times of inductive coin sensors. This method is not particularly precise either.
DE 197 26 449 has made known a method for testing coins by means of an inductively operating sensor assembly in which the primary coil of the coil assembly is fed by a periodic transmission signal containing harmonics. In the known method, a number of switching steps is associated with the transmission signal. Envelope curves are formed from the value of the reception signal during the respective switching steps repeating at the frequency of the transmission signal. An evaluation device forms at least one criterium from the number of the isochronously produced envelope curves to produce the acceptance or rejection signal. Envelope curves are characteristic of the nature of a coin and can be evaluated in an appropriate manner. Thus, for example, the relationship of the amplitudes of the envelope curves is a characteristic measure.
It is the object of the invention to describe a method for measuring the diameter of coins which makes possible a particularly precise determination.